CAIRO (Reuters) - A prominent radical Islamist cleric, who once called for the destruction of the pyramids, has died in prison in Egypt, security sources said on Wednesday.

Morgen Salem was a leading member of Salafiya Jihadiya, a militant group founded by Mohammed al-Zawahiri, the brother of Al Qaeda leader Ayman al-Zawahiri.

Salem, believed to be in his sixties, died of natural causes, the security sources said.

In 2012, Salem appeared on Egyptian television and argued for the destruction of the pyramids and other ancient Egyptian monuments, comparing them to pagan temples.

Egypt faces an Islamist insurgency based in the remote but strategic Sinai Peninsula, and Salafiya Jihadiya was once considered the most prominent militant group operating there.

It has since been overshadowed by Islamic State's Egypt affiliate, which has killed hundreds of police officers and soldiers since Islamist president Mohamed Mursi was removed by the army following mass protests against his rule in 2013.

(Reporting Ahmed Mohamed Hassan, Writing By Shadi Bushra; editing by Ralph Boulton)